What it usually means, the likely causes in order, what to safely try first, what a pro charges in Canada, and when to stop and call one.
This guide covers the common cases. Describe your exact problem and get a personalized diagnosis — free to start, with a DIY vs. call-a-pro verdict and a real cost estimate.
Water pooling near the indoor furnace or air handler, when no plumbing fixture is involved, is almost always the air conditioner or a high-efficiency furnace failing to drain its condensate. Cooling wrings moisture from the air, and condensing furnaces produce water as they burn - all of it is meant to leave through a small PVC drain line or a condensate pump. When that line clogs with algae and sludge, or the pump fails or fills, water backs up and finds the floor, the ceiling below, or the inside of the cabinet. Some systems have a float switch that shuts cooling down when water backs up, which is why a drain problem can also look like a no-cooling problem.
Treat active condensate water as a same-week, often same-day, problem: it is water inside the house, and it keeps coming for as long as the system runs. Turn the system off until the drain is cleared if water is spreading, dripping through a ceiling, or reaching electrical parts. A float switch that has shut the system down is protecting you - do not bypass it to restore cooling while water still stands in the pan.
The most common cause. The slow, dark, nutrient-rich trickle in the drain line grows algae and sludge until the pipe blocks, often at a trap or a low spot. The pan under the coil then fills and overflows. Homes that cool all summer produce enough condensate to flood a pan quickly once the exit is closed.
Where the drain cannot run downhill to a floor drain, a small pump lifts the water out. Pumps clog, their float sticks, or the motor fails, and the reservoir overflows quietly beside the unit. A pump that is silent when its reservoir is full, or that runs without lowering the water, has identified itself.
The pan under the coil catches everything the coil sheds. Metal pans rust through with age, plastic pans crack, and any pan overflows when ice melts suddenly off a frozen coil. Water that appears only during heavy cooling or right after a freeze-thaw episode points at pan capacity or condition.
When poor airflow or low refrigerant freezes the coil, the system may still seem to work - until the ice melts and releases far more water than the drain and pan handle at once. Condensate flooding that follows weak airflow or icing is really two problems: clear the water, then fix the freeze cause or the flood will return.
| Job size | Typical estimate (CAD) |
|---|---|
| Simple fix — the straightforward version of this repair | $120–$400 |
| Major job — complex, hidden, or replacement-level work | $450–$2,500 |
Estimates only, calibrated to Greater Toronto Area trade rates and adjusted by region on this site — not a quote. Your exact price depends on the cause, parts, and access. A personalized diagnosis narrows this range using your specific symptom.
Mopping, photographing, checking a pump reservoir, noting drain routing, and replacing a filthy filter are homeowner steps. Clearing a clogged line properly, replacing a pump, pan, or float switch, and fixing the freeze or furnace fault behind repeat flooding are technician work - especially where drain lines run inside walls or ceilings. Book promptly: condensate damage is cumulative, and ceilings and floors below an air handler are usually what turns a small drain clog into a large repair bill.
High-efficiency condensing furnaces produce water while heating, because they extract so much heat that exhaust moisture condenses inside the unit and must drain away. That winter water uses much of the same drain path as summer air-conditioning condensate, so a clogged line can flood in either season. Tell the technician which season produces the water - it steers the diagnosis.
Very likely. Many systems have a float switch in the drain pan or line that cuts cooling when water backs up, precisely to prevent a flood. The shutdown is the protection working, not a second failure. The fix is clearing the drain or repairing the pump so water leaves; resetting cooling without draining will just trip the protection again or overflow.
Gentle suction at the outside drain outlet is a common homeowner technique and sometimes pulls a slime plug free. Chemicals are less helpful and can damage pans and pumps, and pressure blown from the wrong end can separate fittings hidden in walls. If gentle suction and a clean filter do not restore drainage, a technician with proper access and a brush or nitrogen setup is the safer next step.
Prevention is routine: keep filters clean so coils stay clean, have the drain line flushed and the pan treated during seasonal service, and confirm the line has proper slope with no sagging sections where sludge settles. A system that clogs yearly despite service may have a slope or routing flaw worth correcting once, rather than a cleaning to repeat forever.
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